The present invention relates to an exhaust gas purification system that purifies particulate matters (hereinafter “PM”) from the exhaust gas discharged by diesel and other internal combustion engines using a continuous regeneration-type diesel particulate filter (hereinafter “DPF”).
In the same way as for NOx, CO, and also HC etc., restrictions on the volume of PM discharged from internal combustion engines such as diesel engines grow severe every year. Techniques for collecting this PM in a filter known as a DPF and for reducing the quantity thereof by discharging externally have been developed.
DPFs for collecting this PM include a monolithic honeycomb form wall flow type filter made of ceramic, a fiber form type filter made of fiber shape ceramic or metal, and so on. An exhaust gas control system using one of these PDFs are installed on the way of the exhaust passage of an internal combustion engine, similarly to the other exhaust gas control systems, for cleaning exhaust gas generated in the internal combustion engine before discharging the same.
These DPF devices include a continuous regeneration-type DPF device wherein an oxidation catalyst is installed upstream of the DPF, a continuous regeneration-type DPF device wherein the PM combustion temperature is lowered by the effect of a catalyst supported on a filter with catalyst and PM is burned by the exhaust gas, etc.
The continuous regeneration-type DPF device wherein the oxidation catalyst is installed upstream of the DPF uses the fact that the oxidation of PM by NO2 (nitrogen dioxide) is executed at a lower temperature than the temperature at which the oxidation by oxidizing PM with oxygen in the exhaust gas is executed. This DPF device is composed of an oxidation catalyst and a filter. NO (nitrogen monoxide) in the exhaust gas is oxidized to NO2, by an oxidation catalyst supporting platinum or the like on the upstream side. PM collected by the filter on the downstream side is oxidized by this NO2 to CO2 (carbon dioxide). Thereby, PM is removed.
Besides, the continuous regeneration-type DPF device of filter with catalyst is composed of a filter with catalyst such as cerium oxide (CeO2). In this DPF device, PM is oxidized by a reaction (4CeO2+C→2Ce2O3+CO2, 2Ce2O3+O2→4CeO2, etc.) using O2 (oxygen) in the exhaust gas by means of the filter with catalyst, within the low temperature range (on the order of 300° C. to 600° C.). On the other hand, PM is oxidized by O2 (oxygen) in the exhaust gas, within the high temperature range (equal or superior to the order of 600° C.) which is higher than the temperature where PM is burned with O2 in the exhaust gas.
In this continuous regeneration-type DPF device of filter with catalyst, the oxidation catalyst is also installed on the upstream side. The installation of this oxide catalyst raises the exhaust gas temperature, through oxidation reaction of unburned HC and CO in the exhaust gas, and stimulates oxidation and removal of PM. At the same time, this oxidation reaction prevents the emission of unburned HC and CO into the atmosphere.
However, these continuous regeneration-type DPF devices also cause the problem of exhaust pressure rise by the clogging of this filter. In other words, when the exhaust gas temperature is equal or superior to 350° C., PM collected by this DPF is burned continuously and cleaned, and the DPF regenerates itself. However, in case of low exhaust gas temperature and in an operating condition of an internal combustion engine where the emission of NO is low, for example, in case where the low exhaust gas temperature state such as idling of internal combustion engine, low load/low speed operation continues, the oxidation reaction is not stimulated as the exhaust gas temperature is low, the catalyst temperature lowers and the catalyst is not activated and, moreover, NO lacks. Consequently, the aforementioned reaction does not occur and the filter can not be regenerated through oxidation of PM. As a result, PM continues to deposit in the filter and the filter clogging progresses.
As a measure against this filter clogging, it has been conceived to forcibly burn and remove the collected PM by forcibly raising the exhaust gas temperature, when the amount of clogging has exceeded a predetermined amount. As for means for detecting the filter clogging, there are some methods such as a method for detecting by the differential pressure across the filter, and a method for detecting through judgment of the PM accumulation quantity by calculating the quantity of PM collected from the engine operation state from a predetermined map data. Besides, as means for exhaust gas temperature raising, there is a method by injection control of the injection in the cylinder, or a method by fuel control in the direct fuel injection in the exhaust pipe.
The cylinder injection control executes an auxiliary injection after a main injection at a timing delayed from a normal burn so as to continue the burn at a delayed timing, in the case where the exhaust gas temperature is lower than the active temperature of an oxidation catalyst disposed upstream of the filter or supported on the filter. The exhaust gas is heated by executing so-called multi injection (multi-stage injection) to the temperature higher than the active temperature to execute a post injection (posterior injection) and then the fuel in the exhaust gas is burned by the catalytic reaction of the oxidation catalyst. Thereby the filter is regenerated by burning and removing collected PM after raising the exhaust gas temperature higher than a temperature the PM collected in the filter can be burned.
Normally with continuous regeneration-type DPF devices, as disclosed in Japanese patent application Kokai publication No. 2002-276340 and Japanese patent application Kokai publication No. 2003-286887, for example, when the collected quantity of PM reaches a preset limit, the traveling condition is automatically changed to regeneration mode and the collected PM is oxidized and removed by forcibly raising the exhaust gas temperature or increasing the quantity of NOx. And thereby, the filter is regenerated.
In the case where traveling patterns with a high exhaust gas temperature are generally frequent, such as the case in traveling on expressways is the main purpose of the user, an uneven accumulation of PM which does not appear as a differential pressure of the DPF device is developed at the periphery of the filter. Therefore, another method is conceived which forcibly burn the PM during traveling by supplying a traveling distance into a regeneration starting condition.
Nevertheless, in terms of actual traveling conditions, in particular, in such a place like an urban area, traveling and stopping are frequently repeated because of traffic signals and the like. Therefore the repetition makes internal combustion engine load varies between traveling condition and stationary idling condition in a complicated manner and the exhaust gas temperature which is important for the DPF regeneration control varies also in a complicated manner. For this reason, it is possible dependent on traveling patterns that DPF regenerating control will not terminate within the preset time or the exhaust gas temperature is not raised sufficiently. Therefore it arises a problem that the collected PM is not be burned and removed sufficiently.